Tom Clancy, a prolific writer famous for his best sellers about espionage and the military, died Oct. 1 at the age of 66, ABC News has confirmed with his publisher, Penguin Group.

The cause of death is unknown.

“I’m deeply saddened by Tom’s passing. He was a consummate author, creating the modern-day thriller, and was one of the most visionary storytellers of our time,” Penguin Group executive David Shanks said in a statement. “I will miss him dearly and he will be missed by tens of millions of readers worldwide.”